Big bird had swift legs

An ancient meat-eating 'terror bird' discovered in northern Patagonia has a record-breakingly large head but the big beast could still sprint for its prey.

The fossil discovery is the largest documented example of a group of fearsome birds called phorusrhacids, flightless carnivores that roamed South America some 60 million to 2 million years ago.

The birds earned the moniker 'terror birds' because of their impressive stature and carnivorous palate. They could tower up to 3 metres in height far larger than an ostrich - and had huge, hooked beaks for gouging their prey.

Phorusrhacids were the top carnivores in South America, occupying a niche typically taken by ancient cats, dogs or other mammals elsewhere in the world. They evolved there when the continent was largely isolated from the rest of the world's landmass. "They're like dinosaurs without a tail, and became the mega-predators of the continent," says Luis Chiappe of the Natural History Museum of Los Angeles County, California.

Maximum headroom

The new 14-15-million-year-old Argentine fossil is unusual because its entire skull is intact, the researchers report in Nature1. Chiappe and his colleague, Sara Bertelli, found that its head, at more than 70 centimetres, is 10% larger than that of other phorusrhacid species found thus far, and more boxlike.

Chiappe says that the discovery also challenges ideas about how these monster birds moved. Palaeontologists have generally assumed that larger terror birds were cumbersome and so were forced to move slowly. But the foot bones of the new fossil are slender and long, like those of modern fast-runners such as the emu or ostrich, suggesting that it could have moved at a fair clip.

The monster bird probably chased down its prey and used its beak to crush mammals as big as lambs. "Imagine what an animal with a head this size would have done," Chiappe says.

The closest living relative of the phorusrhacids today are thought to be the seriemas, carnivorous birds living in South America but only half a metre in height.